net:CXGB4: fix leak if sk_buff is not used

An sk_buff is allocated to send a flow control message, but it's not
sent in all cases: in case the state is not appropiate to send it or if
it can't be enqueued.

In the first of these 2 cases, the sk_buff was discarded but not freed,
producing a memory leak.

Signed-off-by: Íñigo Huguet <ihuguet@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>